1. Field of the Invention
This invention relates to a twin-screw extruder with cylinder surrounding the twin-screws of the extruder and a hollow cylinder mantle surrounding the cylinder.
2. Description of the Prior Art
Screw extruders are used in many ways in the processing of plastics. In order to provide a work result of qualitatively high value, it is important that the plastic being processed be extruded in a manner that is as uniform as possible and within an optimal temperature range. The cylinder of the extruder, which is provided with a hole corresponding to the dimensions of the screw unit, which form the extrusion space for the plastic to be processed, is therefore generally pulled through channels in its wall, through which a tempering medium that is capable of flowing (as a rule for cooling, but also for heating) can be fed.
A cylinder for a twin-screw extruder is known from EP 00 42 466 A1, which consists of two coaxial parts, one fed into the other, namely an inner cylinder part, which has a double hole (extrusion space) for the twin-screw unit, shaped like spectacles, and an outer, hollow-cylinder mantle, which lies close to and surrounds the inner cylinder part and with respect to which the forces exerted by the extrusion pressure fulfill a carrier function. In this case, both the inner cylinder part and the hollow-cylinder mantle are pulled through channels running in the longitudinal direction of the cylinder for a tempering medium. In order to be able to cool the extrusion space as effectively as possible also in the region of its thickest wall, namely in the region of the spandrel of the spectacle-shaped hole, cooling channels are arranged in the inner cylinder part in the region of this spandrel. In the nearby region of the planes through the longitudinal axes of the twin-screw unit, cooling of the extrusion space occurs mainly through the cooling channel of the hollow-cylinder outer mantle. This means that, for this, the heat from the inner cylinder-part must flow over into the hollow-cylinder mantle. This is prevented by the separation seam between the two parts of the cylinder. Therefore, the use of heat-conducting pastes has been recommended, with greater or less success, in order to improve the heat transfer in this critical region. From the viewpoint of manufacturing technology, this solution has the disadvantage that cooling channels must be made in both parts of the cylinder. Also, the hollow-cylinder mantle cannot also be used as the inner cylinder of a single-screw extruder, since the cooling channels are concentrated on only one part of its circumference.
Another structure for the cylinder of a twin-screw extruder is known from DE 26 59 037 C3, which likewise consists of an inner cylinder part and a hollow-cylinder mantle. For cooling of the extrusion space, a cooling channel is provided that has a course similar in principle to a helix. The cooling channel is arranged in the region of the surface separating the inner cylinder part and the hollow-cylinder mantle. In this case it is composed of partial pieces of different shapes, in the following manner on the upper side of the inner cylinder part, grooves are etched at equal distances from one another, with vertical walls, in such a way that the bottoms of the grooves are all parallel to the longitudinal axes of the planes in the twin-screw unit, the direction with respect to the longitudinal axis is chosen, however, at exactly the opposite angle. The arrangement of the upper and lower grooves and their distances are selected in such a way that their ends overlap pair-wise. In order to connect the overlapping ends in view of joining the tempering media, sickle-shaped connecting grooves are etched on the inner surface of the hollow-cylinder mantle. The cooling channels generated in this manner, which run like helices, have the advantage that the tempering medium in the region of the spandrel of the spectacle-shaped hole is brought to the extrusion space relatively densely. From the viewpoint of manufacturing technology, creating of the cooling channel still requires a comparably high expense, since not only must the grooves be etched on the upper and lower side of the inner cylinder part, but etching work is also necessary on the inner side of the hollow-cylinder mantle. In addition, the hollow-cylinder mantle produced in this way is only usable for the corresponding inner cylinder part of a twin-screw extruder, but not as an inner cylinder part for a single-screw extruder.
Another solution for a cylinder of a twin-screw extruder that has an inner cylinder part and a hollow-cylinder mantle surrounding it is known from DE-OS 20 61 700, which is the overall state of the art. The wall of the inner cylinder part is cut through with numerous adjacent grooves above, below, and to the side of the spectacle-shaped hole for the twin-screw unit. The bottoms of these grooves run in straight-line segments or in arcs in such a way that the remaining wall to the extrusion space has approximately the same thickness everywhere. This requires a correspondingly high manufacturing cost. In order for connect the individual grooves into a channel running like a helix for the cooling medium, the grooves on the bottom side of the inner cylinder part are connected together in such a way that three partial pieces, two of which run perpendicular to the longitudinal axis of the twin-screw unit and a middle one is directed at an angle to the longitudinal axis, in such a way that an overflow from one winding of the cooling channel to the next results. This in turn leads to an increase in manufacturing costs for the inner cylinder part. Since walls (bridges) remaining between the individual windings of the cooling channel should serve to transfer loads to the hollow-cylinder mantle that surrounds the inner cylinder part, the bridges, which have relatively small dimensions, arranged above and below the spectacle-shaped hole of the extrusion space and were originally especially high, are etched on a surface parallel to the plane of the twin-screw unit. On this etched surface, the bridges are each made in a cylinder section formed from filling material on the upper side and on the lower side of the inner cylinder part, the outer contour of the cylinder section corresponding to the hollow-cylinder mantle. These cylinder sections cover the individual windings of the cooling channels from top to bottom. In this way, the cross-section of the cooling channel, when viewed from above the circumference, remains approximately uniform in size. Although this known construction makes it possible to use a hollow-cylinder mantle with a smooth inner surface, it not only requires an increased manufacturing cost to generate the individual winding of the cooling channel, but also an additional cost for creating the two cylinder segments.
The object of the present invention is to develop a cylinder for a twin-screw extruder so that the manufacturing cost required remains as small as possible, whereby an especially uniform and effective tempering of the extrusion space is to be provided. In addition, the hollow-cylinder mantle of the cylinder should be usable universally, as far as possible. Specifically, it should be usable for single-screw as well as twin-screw extruders.
The object of the present invention is met by a cylinder assembly for a twin-screw extruder having a cylinder with a variable wall thickness which defines an extrusion space for receiving twin screws of the twin-screw extruder. A hollow-cylinder mantle surrounds the cylinder. At least one channel having a helical shape and a cycle depth is formed on an outer side of the cylinder by a winding vortex process. An inlet and outlet are formed in the hollow-cylinder mantle in communication with the at least one channel for allowing a flow of a tempering medium from the inlet to the outlet via the at least one channel. The hollow-cylinder mantle closes the at least one channel. The cycle depth of the at least one channel varies along the circumferential direction of the cylinder such that the cycle depth is largest in regions where the wall thickness of the cylinder is largest and the cycle depth is smallest in regions where the wall thickness of the cylinder is smallest.
The present invention starts from a twin-screw extruder with a cylinder, which surrounds the twin-screw of the extruder, forming an extrusion space. The cylinder is surrounded in turn by a hollow-cylinder mantle, the inner surface of which is made smooth. On the outer side of the cylinder, at least one channel is built in, which extends in the form of a helix in the longitudinal direction of the cylinder and can be connected for conducting a tempering medium. In the state of the art, the channel formed as a helix is closed in the radial direction through a part of the cylinder by separately produced cylinder sections, while the corresponding connection in the other regions of the circumference are closed directly by the hollow-cylinder mantel. In contrast the channels in the cylinder constructed according to the invention is/are closed over the entire circumference by the hollow-cylinder mantel itself. Thus no components are needed that correspond to the cylinder sections from the overall state of the art. A significant cost reduction for the cylinder according to the invention results from the circumstance that the channel(s) for the tempering medium are generated by a winding vortex. This involves an especially simple manufacturing process that can be implemented by placing the piece under tension and generating channels with a completely uniform helical course, which is a great advantage for the flow-through of the tempering medium. The channel(s) thus have no sudden kinks or other flow obstacles.
In principle, the cylinder can be provided with a single channel with a helical shape. In most cases, however, it will be appropriate, such as with multiple windings, to provide several channels next to one other. Advantageously, there will be three or four such channels running parallel to one another on the outside of the cylinder. The pitch of the channel(s) will generally be kept constant over the length of the cylinder. In individual cases, however, it may be reasonable to change this pitch, in order to affect the dwell time of the tempering medium in a particular section of the cylinder along its longitudinal axis. By selecting a large pitch, the tempering medium can be fed in the direction in which the extruder moves or even in the opposite direction, as needed.
In many cases, a rectangular shape may be provided for the channel(s). The depths of each channel in the radial direction, i.e. its cycle depth, may remain constant over the circumference of the cylinder. The advantage is that the wall thickness of the extrusion space is likewise constant over the circumference and thus the flow patch through the wall of the extrusion space for the heat to be transportedxe2x80x94seen over the circumferencexe2x80x94likewise remains constant. In regard to the arrangement of the individual windings of the channel(s) of the cylinder, it has turned out to be appropriate, especially with channels with a rectangular cross-section and a uniform cycle depth, that the width of the channel(s)(measured in the longitudinal direction of the cylinder) be set at in the region of the 0.7 to 1.2 times the thickness of the bridges between channels. The bridges are dimensioned in such a way that they provide directly an adequate support for load transfer to the hollow-cylinder mantle, without special components such as are needed in the cylinder section according to DE-OS 2,061,700.
In an embodiment of the present invention, a design of the channel(s) with various cycle depths is provided over the circumferences. In this case, the cycle depth is selected as small as possible in the regions where the original wall thickness of the cylinder, i.e. the thickness before the channels are made, is smallest. The greatest cycle depth is provided in the regions in which the original wall thickness is greatest. The transition between the largest and smallest cycle depths run completely smoothly and uniformly, because of the winding-vortex process envisioned for producing the channels. For a twin-screw extruder, a cylinder is recommended in which the largest cycle depth of the channel is about 3 to 5 times the smallest cycle depth, especially preferably about 4 times. In this case it may be appropriate to select something other than a rectangular shape and provide a conic expansion in the outward direction for the cross-section in the radial direction. The width of each channel thus increases in the outward direction. This means that the side surfaces of each channel in the section through the length of the cylinder do not run parallel, but include an angle, xcex1. This angle, xcex1, is advantageously in the range of 8-15xc2x0, especially about 10xc2x0. With a conic shape for the cross-section of the channels, it is recommended that the average thickness of the bridges between each pair of adjacent windings in the channel(s) be selected in the range of 1.5 to 4 times the average width of the channel(s). An average bridge width of about 2.5 times the average channel width is preferred.
In order to feed the tempering medium in and out in the simplest possible way, it is advantageously provided that in cylinders with several channels running parallel to one another, a groove going around the cylinder be made in the region of the front ends of the cylinder. The connections for feeding the tempering medium in and out are in this case arranged in a corresponding manner in the hollow-cylinder mantle in the region of one of the two grooves. In this way, the grooves thus receive the function of a distributor or collector for the individual channels, so that the tempering medium can be fed in and out as needed with a single line connection.
Beyond the low manufacturing costs, the cylinder according to the present invention provides an effective heat exchange, since through the effect of the cycle depth of the channels the flow path for the heat can be reduced within the wall of the cylinder as needed. Increased resistances to the flow of heat due to separation surfaces between individual components are avoided in the invention. For a given cylinder diameter, it makes no difference for the design of the hollow-cylinder mantle surrounding the cylinder how many channels, with which cross-section shape, and which cycle depths are provided in the cylinder.